It is known to use a robot to carry out working to a workpiece which is transferred along an assembly line and then positioned at a predetermined location on the assembly line. In this case, if the positioning location of the workpiece varies, accurate robot working cannot be carried out even when the robot is operated at an operating position (teaching point) which is previously taught to the robot.
To this end, conventionally, dislocation of the teaching point attributable to positional dislocation of the workpiece is compensated for on the basis of a correction data, which is calculated based on an actual workpiece position detected by a visual sensor and which indicates the error between the actual workpiece position and a reference workpiece position. However, according to the above-mentioned conventional robot operating method, the visual sensor is essentially required, which entails inconvenience.